The present invention relates to polymers that are thermostable, weatherable and have good flow characteristics. More particularly, the present invention relates to poly(resorcinol phthalate-polycarbonate) copolymers that include soft-block moieties. Such polymers may be advantageously used in multilayer articles as UV-protective coatings.
Various polymeric articles have a problem of long-term color instability. In many cases this instability is seen as yellowing of the polymer or loss of glossiness. Yellowing of polymers is often caused by the action of ultraviolet radiation. Such yellowing is frequently designated “photoyellowing”. To reduce photoyellowing, ultraviolet absorbing compounds (UVA's) may be incorporated into the polymer. For the most part, UVA's are low molecular weight compounds, which must be employed at relatively low levels, typically up to 1% by weight, to avoid degradation of the physical properties of the polymer. For example, incorporation of UVAs may lead to loss of impact strength, loss of high temperature properties as reflected in heat distortion temperature, and/or susceptibility to attack by organic liquids.
Polymers comprising polyesters containing resorcinol arylate chain members often provide good resistance to photoyellowing and loss of gloss, and thus are considered to possess good “weatherability.” The arylate moieties typically contain isophthalate, terephthalate, and mixtures of isophthalate and terephthalate. Polyesters of resorcinol arylates may provide good protection against photoyellowing when coated over a resinous substrate.
The good weatherability properties of polyesters containing resorcinol arylate units is believed to arise in large part from the screening effect these polymers may provide against ultraviolet (UV) light. On exposure to UV light, polymers comprising resorcinol arylate chain members can undergo a photochemically-induced Fries rearrangement that converts at least a portion of the polymer from polyester chain members to o-hydroxybenzophenone-type chain members. The o-hydroxybenzophenone-type chain members act to screen against further UV exposure and protect UV-sensitive components underlying a resorcinol arylate-containing composition.
Polyesters containing resorcinol arylate chain members may be made using melt polymerization or interfacial methods. Polyesters containing resorcinol arylate chain members may be prepared by melt methods as disclosed in U.S. Pat. No. 4,127,560 and in Japanese Kokai 1/201,326. The methods, however, do not allow the incorporation of greater than 30 mole percent terephthalate and were found to result in a polyester having unacceptable levels of color. Polyesters containing resorcinol arylate chain members have also been prepared by an interfacial method (see e.g., U.S. Pat. No.3,460,961; and Eareckson, Journal of Polymer Science, vol. XL, pp. 399-406 (1959)).
One common problem in the preparation of resorcinol arylate-containing polyesters is the presence of destabilizing anhydride bonds, which are believed to be easily broken under thermal conditions typical of polycarbonate processing to produce shorter chains terminated by acid end groups. These acid end-groups may in turn, accelerate the hydrolysis of the arylate moiety, generating additional carboxyl and hydroxyl end-groups, and further contributing to molecular weight degradation and loss of other desirable properties. Methods for preparing thermostable resorcinol arylate-containing polyesters substantially free of anhydride bonds are described in commonly owned U.S. Pat. Nos. 6,265,522, 6,291,589, 6,294,647, and 6,306,507.
The good weatherability properties of polymers comprising resorcinol arylate chain members make them especially useful in blends and in multilayer articles in which the polymers act as a protecting layer for more sensitive substrate components. Multilayer articles containing layers made from resorcinol arylate-containing polyester have been described by Cohen et al., Journal of Polymer Science: Part A-1, vol. 9, 3263-3299 (1971) and in U.S. Pat. No. 3,460,961. However, the coatings developed using this method were thermally unstable and had to be applied by solution coating followed by evaporation of the solvent. Also, Japanese Kokai 1/199,841 discloses a method for coating bottles using a polyester of resorcinol and isophthalic acid. The method was limited, however, in only describing coating of poly(ethylene terephthalate) substrates.
Thus, polymers comprising resorcinol arylate have proven to be very useful materials for weatherable applications, and methods are available for making polymers comprising resorcinol arylate that are both weatherable and, due to the almost complete elimination of anhydride linkages, thermostable. Additionally, this family of polymers also has good solvent resistance, good scratch resistance, and excellent ductility. One limitation of this family, however, is the fact that the melt viscosity is higher than polycarbonates with comparable molecular weights. This limitation can potentially preclude some applications, such as use of the polymer for coating UV-sensitive articles. Also, for injection molding of blends with other polymers or the manufacture of multilayer films made by co-extrusion, a match of melt viscosity is preferred. In addition, manufacture of large or thin-walled parts requires low melt viscosity.
What is needed then, is a way to produce thermostable, weatherable polymers comprising resorcinol arylate with good flow characteristics at the temperatures used for coating. To be cost effective, the method should employ components that are easily obtained and that can be incorporated into existing polymerization protocols.